Abstract. There is a large uncertainty in the relative roles of human land use,
climate change and carbon dioxide fertilization in changing desert dust
source strength over the past 100 years, and the overall sign of human
impacts on dust is not known. We used visibility data from meteorological
stations in dusty regions to assess the anthropogenic impact on long term
trends in desert dust emissions. We did this by looking at time series of visibility
derived variables and their correlations with precipitation, drought,
winds, land use and grazing. Visibility data are available at thousands
of stations globally from 1900 to the present, but we focused on 357
stations with more than 30 years of data in regions where mineral aerosols
play a dominant role in visibility observations. We evaluated the 1974 to
2003 time period because most of these stations have reliable records only
during this time. We first evaluated the visibility data against AERONET
aerosol optical depth data, and found that only in dusty regions are the two
moderately correlated. Correlation coefficients between visibility-derived
variables and AERONET optical depths indicate a moderate correlation (0.47), consistent with capturing about 20% of the variability in optical
depths. Two visibility-derived variables appear to compare the best with
AERONET observations: the fraction of observations with visibility less than
5 km (VIS5) and the surface extinction (EXT). Regional trends show that in
many dusty places, VIS5 and EXT are statistically significantly correlated
with the Palmer drought severity index (based on precipitation and
temperature) or surface wind speeds, consistent with dust temporal
variability being largely driven by meteorology. This is especially true for
North African and Chinese dust sources, but less true in the Middle East,
Australia or South America, where there are not consistent patterns in the
correlations. Climate indices such as El Nino or the North Atlantic
Oscillation are not correlated with visibility-derived variables in this
analysis. There are few stations where visibility measures are correlated
with cultivation or grazing estimates on a temporal basis, although this may
be a function of the very coarse temporal resolution of the land use
datasets. On the other hand, spatial analysis of the visibility data
suggests that natural topographic lows are not correlated with VIS5 or EXT,
but land use is correlated at a moderate level. This analysis is consistent
with land use being important in some regions, but meteorology driving
interannual variability during 1974–2003.